For control of a permanent magnet synchronous motor (PMSM), vector control in which a position of an N pole of the permanent magnet is a d-axis and a direction of a phase advanced by π/2 from the position is a q-axis, a d-axis component and a q-axis component of motor current are separately controlled is generally employed. Also, as permanent magnet synchronous motor control methods, PWM (pulse width modulation) control in which a pulse width (current conduction ratio or modulation factor) of an inverter is adjusted to control an output voltage and PAM (pulse amplitude modulation) control in which an output voltage itself of an inverter is controlled are known. Also, as a method for further increasing a rotation frequency from a state in which an output voltage of an inverter is maximum, voltage phase control (flux weakening control) in which a d-axis current is operated in a negative direction is known.
In order to provide highly-efficient motor control for a wide rotation range, techniques that perform motor control by means of a combination these control methods are known. As such conventional techniques, for example, motor drive devices that control a motor by means of PWM control at the time of a low output and controls the motor by means of PAM control after a conduction ratio instruction value for the PWM control reaches a maximum are known (see, for example, Patent Document 1). Also, as other conventional techniques, for example, motor control methods in which PWM control and PAM control are switched from each other according to a load on a motor and a commutation phase is controlled according to, e.g., a speed of the motor (see, for example, Patent Document 2).